There is a method of obtaining an oxide superconducting thin film by depositing an oxide superconductor on a substrate, which is a conventional technique for putting an oxide superconducting material to practical use.
For example, RE-based superconductors (RE: rare earth element), which exhibit a superconducting phenomenon at liquid nitrogen temperature (77 K) or higher, particularly yttrium-based superconductors (hereinafter referred to as “YBCO”) represented by a composition formula of YBa2Cu3O7−δ, are often used as the oxide superconductor to be deposited.
Oxide superconducting thin films using the RE-based superconductors are expected to be applied to superconducting fault current limiters, cables, and superconducting magnetic energy storages (SMES), and RE-based superconductors and manufacturing methods thereof have drawn a lot of attention.
However, obstacles to practicalization of oxide superconducting materials, including the RE-based superconductors (particularly, oxide superconducting thin films), include difficulty in improving the critical current density characteristics (hereinafter simply referred to as “Jc characteristics”).
For example, the practicalization of superconducting fault current limiters using oxide superconducting thin films requires that a largest possible electric current flow with a resistance of zero, which in turn requires that the Jc characteristics of the oxide superconducting thin films be improved.
Japanese Patent Application Laid-Open (JP-A) No. 2003-37304 discloses a method of manufacturing an oxide superconducting thin film on a sapphire substrate. It is disclosed that, in this method, a CeO2 buffer layer is formed on a sapphire substrate having an R-plane, and then an inclined layer of LaAlO3 is formed on the CeO2 buffer layer, and then an EBCO thin film is formed on the inclined layer, whereby a lattice mismatch between the oxide superconducting thin film and the buffer layer can be alleviated.
In addition, JP-A No. 2002-150855 discloses a superconductor wire rod that includes an oxide buffer layer formed on a surface of a polycrystalline metallic base having a texture oriented in {100}<001> direction, and an oxide superconducting layer formed on the oxide buffer layer, the oxide buffer layer being composed of two layers. A first oxide buffer layer in the oxide buffer layer is a surface oxide layer of the polycrystalline metallic base, and has a surface roughness in terms of Rmax of 0.15 μm or less, and a second oxide buffer layer is formed on the first oxide buffer layer. It is disclosed that, since the oxide buffer layer is composed of these two layers, an oxide superconducting film having stable crystal properties over the entire length in the longitudinal direction thereof can be realized.
Further, JP-A No. 2003-188427 discloses a superconducting element that includes first and second oxide superconducting layers formed on a substrate and forming a Josephson junction, and an oxide interlayer insulating layer formed between wiring portions of the first and second oxide superconducting layers and expressed by (Ca1-xSrx)SnyOz (wherein 0≦x≦0.8, 0.4≦y≦1.1, and 1.6≦z≦3.4). In the superconductive element disclosed therein, the oxide interlayer insulating film has a thickness of about 300 nm or more and a relative permittivity εr of 40 or lower at operating temperatures of about 40 K or lower, and the oxide interlayer insulating film is capable of flat epitaxial growth with the oxide superconducting layer. The superconducting element has a small wiring capacity, and is suitable for high-speed operation.